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Patented Nov. 2, 1926.

UNITED STATES EDWARD H. LANGE, OF NET/V YORK, N. Y.

METHOD AND MEANS FOR STABILIZING AMPLIFIERS.

Application filed May 21, 1925.

This invention relates to thermionic amplifiers, and the object of the invention is to provide simple method and means whereby a thermionic tube or group of tubes used to amplify alternating currents, and particularly high frequency alternating currents, can be prevented from osc llating at one or more natural periods of the amplifier circuits by the production of self-sustained os cillat-ions. When an amplifier produces self-sustained oscillations either by magnetic or capacitive coupling between the input and output circuit of the thermionic tube or tubes, the output current is not under the complete control of the impressed signal electro-motive-force, but contains alternating currents of undesirable frequencies superposed on the amplifier current of signal frequency, which results in overloading and poor reproduction of the impressed signal wave-form.

As an aid in understanding this invention, the following facts concerning thermionic amplifiers are cited, with particular reference to amplifiers for high frequency alternating currents. In a thermionic amplifier, coupling in general exists between the input and output circuits, which may consist of mutual inductance or mutual capacity, or both. By means of this coupling an electro motive-force is impressed upon the input circuit due to alternating currents in the output circuit. and when amplified, sufficient electro-motive-force may be produced in the output circuit to sustain the output alternating current. An equilibrium condition is reached when the output alternating current reaches a magnitude such that the output electro-motive-force required to sustain this current is just equal to the amplified input electro-motive-force derived from the mutua-l impedance of the input and output circuits. Although the mutually induced electro-motive-forces from the output to the input circuit of the amplifier can be practically eliminated by magnetic shielding and proper or entation of the magnetically inductive elements of the circuit, capacitive coupling between the input and output elements of the thermionic tubes still exists, and the present invention deals with a method of annulling Serial No. 31,758.

the mutual capacitive electro-motive-force between output and input elements of the ampl fier or of intermediate stages of the amplifier by establishing through another capacitive coupling an electro-niotive-force equal in magnitude and opposite in phase to the former.

Another important feature of this invention is the effective use of metallic sh elding for the stages of the amplifier in producing a stabilizing action in conjunction with the inter-electrode capacity. lVhile the use of shielding is not essential to this invention, where more than two stages of radio he quency amplification are required a definite advantage results from the combined action of the shielding capacity to the input circuit and the inter-electrode capacity. The application of these ideas to radio frequency thermionic amplifiers having unavoidable capacitive coupling between input and output elements is illustrated in the accompanying drawings.

Figure 1 is a diagram of an ampl fier hav- 7'5 ing means to prevent self-sustained oscillations due to inherent capacitive coupling between grid and plate.

Figure 2 illustrates diagrammatically the electric c rcuit equivalent of one of the stages of Figure 1.

Figure 3 is a vector diagram which represents the currents and electro-motive-forces of the circuit of Figure 2, in proper phase relation, and shows how stabilization is produced.

Referring to Figure 1. a radio frequency clectro-1notiveforce to be amplified is impressed upon the input circuit 2, 3y either directly, or by means of the antenna 1. The output circuit of the first thermionic tube is energized through a radio frequency choke coil 11 which has a high impedance compared with the bypass condenser 8, thereby effectively preventing any flow of alternating current through the circuit 9, +13, 11, 10, 7, 9. This circuit, however, permits the flow of a steady current from the filament 9 to the plate 7. The alternating current output circuit consists of the tube impedance 7, 9, a condenser 8 and the output impedance 10/ Then a signal electromotive-force E is impressed upon the grid circuit of the first tube, an alternating electro-motive-force of magnitude ,LLE is es" tablished in the outputcircuit 7, 10, 8, 9, 7, ,a being the voltage: amplification factor of the tube, and a corresponding alternating current set up through the capacitive rcactance 8 and inductive reactance 10. The capacitive reactance 8 is so proportioned to the inductive reactance 10 that the latter is considerably in excess over the frequency range for which stabilization is to be etfected. The resultant in'ipedance from lilament 9 to plate 7 is therefor composed oi? inductive reactance, and the alternating voltage from plate to filament is suhstantially out of phase with the alternating current. The current lags the plate to lilament voltage when the sense of rotation is that shown in Figure 3. The same plate to filament current flows through the bypass condenser 8; the alternatii'ig voltage across the condenser 8 is therefore substantially 90 out of phase with the current; the current leading the condenser voltage in the sense of rotation shown in Figure 3. The voltage from filament 9 to plate: '4' is theretore substantially 180 out of phase with the voltage from filament 9' to condenser 8. Now, the undesirable interaction or" the out put and input circuit of the first tube, Figure1,isdue to the inherent capacity d within the thermionic tube and to small capacities between the circuit connections to the tube grid and plate. The resultant capacity together with the input circuit 2, 3, constitutes a shunt path across the filtlll'lfilli; to plate alternating voltage, and thereby impresses a portion of this voltage upon the grid, of proper phase and magnitude to reinforce the original signal electro-motivcforce, or to give rise to self sustained oscillations independent of the impressed signal. My invention overcomes the above described action by connecting a condenser from the lay-pass condenser 8 to the input or grid element, thereby impressing upon the input or grid to filament circuit a voltage of opposite phase to that impressed through the tube capacity, this stablizing voltage being made of sutlicient magnitude to annul; the opposing voltage. By means of the circuit showi'i in Figure l the grid to filan'ient electro-motive-f0rce can be kept at substantially zero value so far as reaction of the output on the input is concerned. The stal'iilizing action of the condensers 5, 15, etc, can be still but ter understood by reference to Figure 2 and a corresponding vector diagram in Figure 3. The plate inpedance X and X in series form one branch of a. parallel re so nant circuit, the other branch being COD/1' posed of the mutual capacity reactance X and grid to filament input circuit reactance in series. The parallel circuit is supplied neoaoaa through the internal tube impedance T between the filament c and plate P. X is the stablizing capacity reactance connected from the grid G to the high. potential side of the series output capacity reactance X Tn Figure 3, I is the alternating current component of the plate current. This current gives rise to the voltage drop E across the capacity reactance K and a voltage drop E across the inductive reactance k Due to considerable excess 01' inductive reactance K the voltage drop E is always greater than the condenser drop E k, and the resultant voltage E is always inductive, and in phase opposition to E The voltage E is divided into two parts by the inherent tube capacity reactance X and the stablizing capacity reactance X and the corresprfiiding voltage drops are E and E g respectively. By adjusting the capacity reactance X in relation to the reactance Iii and 55. the voltage drop across 'gD can be made equal in magnitude and opposite in phase to the voltage drop across ternating voltage Zero, and eliminating the reaction from the output to the imput circuit.

What is claimed is:

1. In an amplifier comprising a thermionic device having a cathode, anode, control electrode, and inherent capacity between electrodes, an input circuit connected from said cathode to said control electrode an output circuit consisting of a condenser and inductance in series connected from said cathode to said anode, a stablizing condenser connected from said control electrode to a junction point in said output circuit between said condenser and inductance whereby an electroanotivetorce may be applied to said control electrode ct opposite phase and equal magnitude to the electron]otive-torce applied to said control electrode from said anode by virtue of said inherent capacity, said opposite phase being derived troin a capacity reactance in series with said output circuit.

2. The method of preventing selt sustained oscillations in a thermionic amplifier having cathode, anode, control electrode and inherent capacity, an input circuit and an inductive output circuit, Wl'llCll comprises clectrostatically impressing upon said con trol electrode a reactive electroanotive-torce of opposite phase to the electro-motive-torce impressed upon said control electrode -from said anode, said reactive @lQCtl'O-lHOtlVQ- force of opposite phase being introduced in series with the inductive output circuit, and proportional to the alternating current in said output circuit.

3. in a thermionic amplifier having a cathode, anode, control electrode, a reso nant input circuit, and an output circuit thereby making the gridto lilau'ient alcontaining the primary of a coupled circuit, the circuit connections for preventing reinforcement of oscillations by annulling the etlect of inherent capacity between electrodes which consist of connecting a condenser in series with said output circuit between the cathode and one terminal of the output circuit primary and connecting a stabilizing condenser from said control electrode to said series condenser at the junction of said condenser and coupled circuit primary, thereby impressing upon said control electrode an electro-motive-torce of equal magnitude and opposite phase to the electro-motive-torce impressed upon said control electrode from said anode, and annulling the reaction between said output and said input circuits.

+2. In a thermionic amplifier, an input circuit, a primary output circuit inductively coupled to a secondary circuit and inherently capacitively coupled to said input circuit, means responsive to the current in said primary output circuit, said means establishing in said primary output circuit two unequal electro-motive-torces in. phase opposition, and means applying said unequal electro-motive-jtorces to said input circuit to annul the reaction of said output circuit upon said input circuit.

5. The method or preventing regeneration in a thermionic amplifier having a vacuum tube with cathode, plate and grid, an input circuit, and an output circuit containing the primary of a coupled circuit inherently capacitively coupled to the input circuit through the inherent plate to grid capacity, which comprises electrostatically bridging the tern'iinals of the output circuit primary to the input circuit, and introducing into the primary output circuit an electroinot ve force of opposite phase to the potential ditterence across the electrostatic bridge, and proportional to the output circuit alternating current, thereby balancing the introduced electron'iotive force against the portion of the potential di'llercnce of opposite phase across the output circuit primary which is impressed upon the input circuit, and annulling the reaction (it the output circuit upon the input circuit.

6. In a thermionic radio frequency amplifier comprising a vacuum tube having a cathode, grid and plate, and inherent capaeity trom said grid to said plate, an input circuit, and an output circuit, reactive voltage drop means responsive to the current in said output circuit and establishing in said output circuit a voltage drop from said cathode to an intermediate point in said output circuit, and a second means establishing a greater reactive voltage drop in opposite phase from said intermediate point in said output circuit to said plate, adjustable reactive voltage drop opposing means opposing the reaction upon the grid of the first said reactive voltage drop from the cathode to said intermediate point, and the second said reactive voltage drop from said intermediate point to the plate, and separating means for the alternating and continuous current of said output circuit, said means combining to annul the electro-motive-torce impressed upon the input circuit due to the reaction of the output circuit on the input circuit.

7. in a thermionic radio frequency ainpliiicr comprising a vacuum tube having a cathode, grid, and plate, and inherent capacity from said grid to said plate, an input circuit and an output circuit, separating means for the alternating and continuous current at an intermediate point of said output circuit, means responsive to the output current establishing a reactive voltage drop from said cathoce to said intermediate point and a greater reactive voltage drop of opposite phase from said intermediate point to said plate. and a second means applying the react ve voltage drop from said cathode to said intermediate point, o said grid to annul the reactive voltage drop from said intermediate point to said grid.

8. In a thermionic radio frequency amplitier comprising a vacuum tube having a cathode. a plate. and a grid. and having inherent capacity between said plate and said grid, an input circuit, an alternating current output circuit, and a continuouscurrent output circuit, the method of annulling the reaction of the alternating current output circuit upon the input circuit due to said inherent capacity. which con: oi establishing in said alternating current outputcircuitav reactive voltage drop -t'roin said cathode to a junction between said altcrnat ing current circuit and said continuous current circuit, and a greater reactive voltage drop of opposite phase from said junction to said plate, and impressing upon said grid said first mentioned reactive voltage drop to annul the voltage impressed upon said i rid from said plate by said inherent capacity.

9. ln munbination a three electrode vae uum tube amplifier having an inherent rcgencrative tendency, means tor opposing said regemn'ative teiulency comprising til cuit connections including a condenser and output trans'i'ormer in series in the alternating current output circuit, a radio trcquecv choke coil in the continuous current output circuit, and circuit connections including a second condenser iniprcssine' the reactive drop across said series condenser, upon the input circuit in tnipt ni to the reactive drop impressed upon the input circuit by means of the output transformer 10. In a thermionic amplifier having a vacuum tube with cathode, plate and grid. and inherent capacity between the plate and the grid, a tuned input circuit, an inductive output circuit and a plate energizing circuit, means separating the alternating and continuous plate current, electrostatic bridgingr means including; said inherent capacity, luilgingr said indimtive output circuit, uid alternating current responsive means (K111: nccted between the cathode and the inductive output circuit establishing a potential. drop in opposition. to a part oi the potential drop oi? said electrostatic bridging; means, said means coi'ubining to annul the reaction of the inductive-i output circuit upon the tuned input circuit.

11. The method of: annulling the electrostatic reaction through the inherent plate to grid capacity, of the inductive output circuit uponthe tuned input circuit of a thermionic amplifier having cathode, plate and grid, which comprises introducing into the output circuit between the cathode and the inductive portion ofthe output circuit, an electroniotive force of opposite phase to the alternating potential differenceacross the inductive portion oithe output circuit, and si'allcr than this potential difierencc, and electrostatically coupli the oppositely phased; electromotive force to the input circuit, thereby opposing the introduced electroinotive force against the potential dill'ercnce established across the said electrostatic coupling due to the shunt path formed by the inherent plate to grid capacity and saidclectrostatic coupling in series across the inductive portion of the output circuit.

12. lhe n'iethod of preventing;- self-sustained oscillations due to inherent rupacitire coupling" oi the inductive output circuit with the inductive input circuit in a thermionic amplifier ha'viiu; a cuthooe, plate and grid which comprises clcctrostaliically bri ring the terminals of the inductive portion of the output circuit by a double arm bridge, to the input circuit, one of the arms inchidinu' the inherent plate to grid capacity, and balancing the potential difference across the other of said arms established by the reaction of the inductive output upon the inductive input circuit against a potential difference oi, opposite phase, introduced in series with the output circuit between the cathode and the inductive portion of the output circuit. thereby reducing to zero the electroniotive force established in the input circuit by the output circuit.

1,3. In an electrical network having a first circuit containing; an inductive react-once X carrying a hiq'h frequency alternating; current, and hav' one terminal oi the inductive reactuuce conductively connected to a second circuit, and the other terminal capacitively connected to a point in the second circuit by capacity reactunc K than K the method oi"? anuulling the eleccoaoaa 1.4. In a thermionic amplifier having a vacuum tube with cathode, plate and grid, and inherent capacity between plate and ,Qrrd, an inductive input circuit, an inductive output circuit, and a plate energizing circuit. means separating the alternating and continuous plate current, electrostatic potential divider means including the inherent capacity, dividing the potential drop across said inductive output circuit into two parts, and alternating); current responsive means between said cathode and said inductive output circuit establishing a potential drop in opposition to the potential drop across one o'li' said parts, said means combining upon adjustment to annul the reaction of the in ductive output circuit upon the input circuit.

15. The method of preventing transfer of energy from a first inductive circuit carrying a high frequency alternating current and having an inductive voltage E to a second circuit having inherent electrostatic capacity with the first circuit and a conductive connection with the first circuit, which consists of introducing a reactive voltage E smaller than E and of opposite phase, in series with the first circuit, between the conductive connection of the first and second circuits and inductive portion of the first circuit, and electrostatically coupling the reactive voltage Eu; with the second circuit, thereby opposing" the voltage impressed upon the second circuit by the inductive voltage E through said inl'ierent capacity, the voltage E across the inherent capacity and voltage lfi across the said electrostatic coupling conforming upon adjustment of said electrostat c coupling: and said introduced reactive voltage to the relation:

gp ck EIHD+EQD Eeb 16. T he method of preventing transfer ot energy from the inductive output circuit to the inductive input circuit of a thermionic amplifier having a cathode, plate and grid, and inherent capacity between the plate and grid, which comprises introducing an electromotive force E in series with the output circuit, ot opposite phase to, and smaller than the inductive electromotive force 16, of the inductive output circuit, and electro statically coupling the introduced electromotive force ll to the input circuit. the condition for no transt'er oi energy from the output circuit through the inherent capacity to the input circuit consisting ot the interrelation of the voltage E across the inherent plate to grid capacity, and the voltage l l across the electrostatic coupling obtained by adjustment of the electrostatic coupling and introduced eleetromotive force which conforms to the relation:

lira. t

17. In a thermionic amplifier having a cathode, plate and grid, and inherent capacity between the plate and the grid, a first rcactive circuit means connected between the cathode and plate, and a second reactive cir cuit means connected from an intermediate junction of said first reactive circuit means to the grid, said first reactive circuit means establishing from the i thode to said junction, and from said junction to the plate two unequal voltages in phase opposition, said second reactive circuit means impressing upon the grid a traction of the larger of said unequal voltages equal to the smaller of said unequal voltages, thereby reducing the resultant voltage impressed upon grid due to an alternating voltage between the cathode and plate to Zero, and preventing transfer of energy from the plate to the grid through said inherent capacity.

18. In a thermionic amplifier having a cathode, grid and plate, and inherent capacity between the grid and plate, means dividing the alternating voltage between the cathode and the plate into two unequal voltages in phase opposition, and means impressing a portion of the larger of said unequal voltages equal to the smaller, upon the grid, thereby reducing to ZClO the resultant voltage between the grid and the cathode due to the alternating voltage between the cathode and the plate, said first mentioned means consisting of capacitive and inductive reactances in series, and said second means consisting of capacitive reactance.

19. In a thermionic amplifier having a cathode, grid and plate, and inherent capacity between the grid and the plate, a first reactive circuit means consisting of a capacitive and an inductive reactance in series be tween the cathode and the plate, dividing the alternating voltage between the cathode and plate into two unequal voltages in phase opposition, and a second reactive circuit means consisting of acapacitive reactance impressing a part of the larger of said an equal voltages equal to the smaller of said unequal voltages, upon the grid, thereby preventing the establishment of a resultant voltage between the cathode and grid by the alternating voltage between the cathode and the plate, through said inherent capacity.

20. In a thermionic amplifier having a cathode, control electrode and anode, and inherent capacity between the anode and the control electrode, reactive circuit means comprising three arms containing reactance connected from the cathode, control electrode and the anode to a common junction, dividing the alternating voltage between the cathode and anode into two unequal voltages in phase opposition, and impressing apart of the larger of said unequal voltages equal to the smaller upon the control electrode, thereby preventing the establishment of a resultant voltage between the cathode and control electrode through said inherent capacity by the alternating voltage between the cathode and the anode.

21. In a thermionic amplifier having a cathode, control electrode and anode, and in herent capacity between the anode and the control electrode, a first reactive circuit means bridging the anode and control electrode, and a second reactive circuit means connected from an intermediate point of said first means to the cathode, impressing upon the control electrode an equal voltage of opposite phase to that impressed upon the control electrode from said intermediate point by the first reactive circuit means, thereby impressing upon the control electrode a resultant of zero voltage from the anode, and preventing the production of self-sustained oscillations in the amplifier due to the inherent capacity.

In a system 01 two electric circuits having a conductive connection at one point, and inherent capacity between a terminal of the first circuit and a terminal of the second circuit, a first reactive circuit means establishing in series between said conductive connection and said first circuit terminal two unequal voltages in phase opposition, and a second reactive circuit means impressing upon said second circuit terminal a part of the larger of said unequal voltages equal to the smaller, thereby producing a resultant voltage of zero between the conductive connection and the second circuit terminal, and preventing the transfer of energy from the first circuit to the second circuit through the inherent capacity.

23. In a multi-stage thermionic amplifier, each stage having a thermionic tube with cathode, grid and plate, and inherent capacity between the grid and the plate, a first reactive circuit means establishing in series between the cathode and the plate, two unequal voltages in phase opposition, and impressing the larger of said unequal voltages upon the succeeding stage, and a second reunequal voltages equal to the smaller, upon the grid, thereby annulling the resultant voltage produced between the cathode and the grid throughthe inherent heapaclty by a Voltage between the cathode and the plate,

and preventing in each stage the production of self-sustained oscillations. 1

In Witness whereof I have hereunto set my hand this 20th clay of ll lay, 1925.

EDWARD H. LANGE. 

